1. Field of the Invention
The present invention relates to a quantum information processing method involving a magnetic-field application method for a physical system, in which coherence time is extended by the magnetic field applied to the system, to effectively utilize the extended coherence time in a quantum information processing device that uses a resonator mode. It also relates to a quantum information processing device capable of effectively utilizing the coherence time.
2. Description of the Related Art
In quantum information processing devices such as quantum computers, information (quantum bit information) is represented by superposed states related to a certain physical state of a physical system, such as an atom, ion or photon. In this case, information processing is realized by iterating, for example, individual operations of quantum bits, or conditional gate operations in which interaction between a pair of physical systems is introduced so that a change in a quantum bit corresponding to one of the physical systems will cause a change in a quantum bit corresponding to the other physical system.
It is necessary for each physical system to hold the coherency of its physical quantity during information processing. Accordingly, physical systems with a long coherence time are required. Coherence time means the time until coherence is lost, and hence is also called de-coherence time. The coherence time of the hyperfine structure level of rare-earth ions dispersed in an oxide crystal is exceptionally long for a solid, and can be controlled using electromagnetic radiation of near visible-light frequencies. This being so, rare-earth ions are highly promising as physical systems that enable a quantum information processing device to be constructed using a solid material (see, for example, K. Ichimura, K. Yamamoto and N. Gemma, Phys. Rev. A 58(5], 4116(1998); and K. Ichimura, Opt. Common. 196, 119(2001)).
Further, a dominant method for extending the coherence time of the hyperfine structure level of crystalline rare-earth ions has recently been proposed (see, for example, E. Fraval, M. J. Sellars, and J. J. Longdell, Phys. Rev. Lett. 92(7), 077601(2004)). It has been confirmed experimentally that this method can achieve a remarkable increase in coherence time. However, in general, the de-coherence that can be considerably suppressed at a time is only that between a pair of energy states.
When using crystalline rare-earth ions in a quantum information processing device, the use of a resonator mode is almost indispensable at present. However, in the case of using the resonator mode, no specific magnetic-field application methods are known, which clarify, for example, between which energy states de-coherence should be suppressed, or when a magnetic field should be applied, in order to actually utilize the effect of suppression of de-coherence by magnetic-field application in the quantum information processing device.